The present invention relates to a multistage piston compressor used in, e.g., a vehicular air-conditioning system.
Japanese Unexamined Patent Publication No. Hei 10-184539 discloses a conventional multistage piston compressor. This kind of compressor is provided with a rotary shaft, which is rotatably supported in a case. A valve plate is provided in the case. The valve plate has a plurality of discharge ports and suction ports. A plurality of bores are arranged at predetermined intervals on a circle, the center of which is on the axis of the rotary shaft. A reciprocating piston is housed in each bore. Each piston is connected with a swash plate by a pair of shoes. When the rotary shaft is rotated, the swash plate rotates. The rotation of the swash plate is converted into reciprocating motion of the pistons in the bores by the shoes. A connecting passage connects the discharge port of one bore with the suction port of another bore. A refrigerant passes through a plurality of cylinder bores successively via the connecting passage and is compressed in a multiple stages.
Between an end face of the pistons and the valve plate, compression chambers are defined in the bores. When the difference between the pressure in one of the compression chambers and the pressure in a crank chamber is large, the refrigerant is likely to leak through the gap between the bore and the piston. As a result, since a large amount of blow-by gas, or leakage loss occurs, the performance of the compressor falls.
When the difference between the pressure in the compression chamber and the pressure in the crank chamber is large, the difference between the pressure acting on the front face of the piston and the pressure acting on the rear face of the piston is large. In this case, the piston receives a large compressive reaction force. The compressive reaction force produces a large frictional force between the shoes and the swash plate and between the shoes and the piston. Furthermore, the reaction force acts also on the rotary shaft, to which the swash plate is fixed. Therefore, a mechanical loss is generated and the performance of the compressor falls.
An object of the present invention is to provide a multistage piston compressor that decreases the leakage loss and the mechanical loss.
In order to achieve the above object, the present invention provides the following multistage piston compressor: The compressor includes a case, a suction chamber, which is provided in the case and the internal pressure of which is a suction pressure, and a discharge chamber, which is provided in the case and the internal pressure of which is a discharge pressure. A rotary shaft is rotatably supported in the case. A valve plate is provided in the case. The valve plate includes suction ports and discharge ports. A plurality of bores are provided at predetermined intervals about the axis of the rotary shaft. Pistons are housed in the bores and reciprocate therein in accordance with the rotation of the rotary shaft to compress a refrigerant. A connecting passage connects the discharge port of a specific bore with the suction port of another bore. The refrigerant passes through a plurality of bores via the connecting passage and is compressed in a multistage manner. A compression chamber is defined between an end face of each piston and the valve plate. Pressure setting means sets the pressure acting on the rear face of the piston to an intermediate pressure between the suction pressure and the discharge pressure.